This invention relates to an electronic cut-out for the protection of the output stage transistors of a low frequency amplifier.
Normal fine fuses are practically unusable, on account of their inertia for the protection of power transistors in low frequency output stages so that electronic protective circuits which react substantially more quickly, have to be used.
Inter alia current limiting circuits are known in which on achieving a specified maximum current in the output stage transistors, a transistor controlled by this current becomes conductive and in turn limits the control signal of the output stage transistors to be protected, so that the collector current cannot rise further. The disadvantage of this current limiting circuit is the high power loss occurring in the limiting case, in the output stage transistor, from which considerable thermal problems result.
In the case of other known circuits, the operating point of the circuit is changed. By a change of the direct current quiescent operating point however, disturbances can occur in the circuit, by which the output stage transistors are destroyed in spite of the cut-out present.